1. Field of the Invention
This invention relates generally to processing of Silicon-On-Insulator (SOI) devices, and more particularly, to a method of forming a frontside contact to the silicon substrate of an SOI wafer.
2. Discussion of the Related Art
As is well known, the presence of a semiconductor substrate directly under active devices provides operational limitations on those devices. SOI technology, which provides an insulating layer between the active devices and the semiconductor substrate, has been shown to provide many advantages over technology wherein such insulating layer is not included. For example, these advantages include higher speed, lower power consumption and better resistance to radiation upset.
One of the problems faced in using SOI technology is to provide a bias or ground to the silicon substrate, through electrical contact with the substrate. While electrical contact can be made to the backside of the substrate, extra processing on the backside of the wafer is required to achieve this.
U.S. Pat. No. 5,314,841 issued to Brady et al. on May 24, 1994 addresses these problems by providing a frontside contact to the semiconductor substrate of an SOI wafer. However, this approach is dependant on a relatively complicated process of opening and covering trench regions so that the lightly doped semiconductor substrate may be implanted with ions in order for it to become more heavily doped. While frontside contact to the substrate is provided, this disclosed process requires numerous masking and patterning steps to achieve the desired result, adding to the complexity and expense of the process. It would be highly advantageous to provide an effective process for achieving such frontside contact to the substrate which does not require the complicated masking and patterning steps disclosed in that patent.
In the present invention, in the environment of an SOI structure on a semiconductor substrate, a method for making frontside contact to the substrate is provided. A trench is etched in the SOI structure to expose a surface of the substrate. The etching causes the exposed surface of the substrate to be relatively rough. Next, an insulating layer is formed (for example SiO2 is grown or deposited) over the exposed surface of the substrate, this insulating layer being thin and irregular because of its formation over the relatively rough etched surface. Then, contact material is provided in the trench, and an electrical potential is provided across the contact and substrate sufficient to increase the conductivity of the insulating layer. Prior to providing the insulating layer, nitrogen may be implanted into the exposed surface of the substrate to slow subsequent growth of the insulating layer, resulting in an even thinner insulating layer, i.e., one even less resistant to breakdown upon application of electrical potential across the contact and substrate. In situations where the insulating layer is sufficiently irregular, or is sufficiently thin, an ohmic contact may be achieved between the contact and substrate without the application of electrical potential thereacross. As yet another approach, prior to forming the insulating layer, the exposed surface of the substrate and the wail of the trench are fabricated such that they meet at an abrupt angle. Then, when the insulating layer is provided, for example by growing, the insulating material which forms in the corner defined by the abrupt angle is of poor quality, lending itself readily to breakdown upon application of electrical potential across the contact and substrate.
The present invention is better understood upon consideration of the detailed description below, in conjunction with the accompanying drawings. As will become readily apparent to those skilled in the art from the following description, there are shown and described embodiments of this invention simply by way of the illustration of the best mode to carry out the invention. As will be realized the invention is capable of other embodiments and its several details are capable of modifications and various obvious aspects, all without departing from the scope of the invention. Accordingly, the drawings and detailed description will be regarded as illustrative in nature and not as restrictive.